1. Field of the Invention
The invention relates to the sealing-off of the rotor of hydraulic turbomachines, such as turbines, pumpturbines, accumulator pumps or other pumps, with respect to the turbine casing.
2. Discussion of Background Information
Kaplan turbines for low, Francis turbines for medium and Pelton turbines for high fall heights form the modem standard repertoire in the field of turbine construction. Francis turbines in this case cover essentially the fall-height range of between 30 m and 400 m.
In this context, Francis turbines reach efficiencies of about 95% in the lower fall-height range and of up to more than 92% in the upper fall-height range. Particularly in the upper fall-height range, the gap losses and disc friction are responsible for the decrease in efficiency which it has hitherto been impossible to eliminate. To explain these two phenomena, the construction and operation of a Francis turbine will be dealt with briefly below:
In Francis turbines, the water driving the turbine flows out of a horizontally lying spiral through a guide wheel to the rotor. The rapidly rotating rotor converts the pressure and velocity energy of the water into the rotational movement of the shaft, on which the rotor is fastened, and consequently drives a generator for current generation. The driving water leaves the rotor and also the turbine through a suction pipe downwards in the axial direction.
In the peripheral region of the rotor, at the outer ends of the blade ducts, the latter move at high speed past the stationary turbine casing, and, between these parts, it is not possible to avoid a gap, through which the water coming from the guide blades flows past the rotor and thus passes into the gap-like regions between the outer surface of the rotor and the inner surface of the turbine casing. Considerable frictional losses occur due to the high speed differences between the stationary casing and the rotating rotor. Furthermore, the high pressure prevailing in the upper gap generates a powerful axial thrust which subjects the shaft and the axial bearing to extreme load. For this reason, a labyrinth seal is provided in the outer circumferential region of the rotor and the water passing through this labyrinth seal is led past the turbine. The prior art thus accepts a leakage which, even in medium-sized turbines, may amount to 0.5 m3/s.
Since, then, for the reason mentioned, the labyrinth seal is arranged in the outer region of the rotor, the small gap widths which are sought after give rise to considerable frictional losses and high braking torques. Furthermore, these seals are costly to produce and, precisely also because of the high relative speeds between the surfaces located opposite one another, are exposed by the impurities repeatedly entrained and contained in the water, such as sand grains, wood fragments and the like, to constant wear which makes complicated maintenance work and repairs necessary.
It is not possible to provide an actual seal in the outer region of the rotor in any way other than directly on the shaft which, of course, is led through the casing. The reason for this are, on the one hand, the high relative speeds, already mentioned several times, of the components located opposite one another and, on the other hand, the dynamic problems which arise due to the unavoidable relative movements (transversely to the main rotational movement) in the case of these dimensions and the forces which occur. These relative movements take place essentially in the axial direction and arise in the event of changes in the operating state, but also due to tolerances, bearing play, randomly excited vibrations and the like.
In the electricity generation, then, the question of as high an efficiency as possible is of critical importance, on the one hand, because of commercial considerations and, on the other hand, for reasons of environmental protection. Of the abovementioned 5 to 7% of the energy currently not yet utilized and contained in the driving water, a comparatively large fraction, particularly in the case of Francis turbines operating in the range of high fall heights and consequently pressures, is due to the gap losses and here, in particular, again due to the losses in the upper gap region, in conjunction with the accompanying disc friction.
Various attempts to deal with this problem have already been undertaken. In this respect, reference may be made merely to a proposal which was published under the definition “Polar Sealing” by VA TECH VOEST MCE described in (EP 1 098 088) and in which, in the outer region of the rotor, from the casing outwards, an ice bead is formed by cooling, which, during operation, grows as far as the rotor and comes to bear there in a slightly abrading manner and thus assumes sealing. This is an outstanding example of how difficult it is to seal off in this region of a Francis turbine when one of the leading international companies in the field of the production of turbines of this type proposes such a complicated self-regenerating seal.
The problems associated with this seal are, above all, the risk of breakage of at least part of the ice ring and the subsequent leak, which is why the publication proposes to provide this seal in addition to the traditional labyrinth seal. Although a reduction in leakage and in the problems connected with this can be achieved by means of this strategy, this is nevertheless at the expense of high investment and the use of a complex additional component which requires additional maintenance and care.
A solution with hydrostatic mounting is known from DE 25 54 217 A1 (corresponds to U.S. Pat. No. 4,118,040 from the Search Report): in this case, a sealing ring is held via essentially tangentially running arms and is mounted sealingly in an annular groove of the casing. This sealing with respect to the casing may take place via elastomeric rings or similar elements which are mounted in the groove and which come to bear over a large area on the outer surfaces of the ring, thus, in turn, markedly obstructing the moveability of the latter in the axial direction and thus adversely impairing the change in the gap height between the ring and the rotor. However, in view of the unavoidable axial movement of the rotor with respect to the casing, this change is absolutely necessary in order to achieve as efficient a hydrostatic seal as possible. In a number of exemplary embodiments, the water required for hydrostatic sealing is supplied via tubes or the like, thus further obstructing its moveability.
Another solution is known from CH 659 856 A5: a ring which is essentially immovable with respect to the casing is sealed off with respect to the rotor (hub disc, cover disc) radially and in a non-contact manner by means of hydrostatic sealing, whilst, to improve the rapid adjustability of the gap height, the ring is mounted with as little friction as possible in the axial direction likewise by means of a type of hydrostatic mounting. The bearing water for the axial bearing is in this case branched off from the bearing water for the radial bearing. However, this ring is unavoidably also held on (a plurality of) radially running cylindrical supply lines for the bearing water and is sealed off with respect to these lines by means of O-rings. This mounting of the ring therefore cannot be designated as “floating”, since the change in the gap height in the radial bearing is markedly obstructed by these O-rings. The entire construction of the seal is complicated and makes it necessary to adhere to a whole series of narrow tolerances on various components which have considerably large dimensions.
DE 196 11 677 A1 proposes a seal, designated as “non-contact”, with a ring, designated as “floating”. The ring is in this case mounted on the casing sealingly and in a rotationally fixed and elastically supported manner (and not non-contact), and the cylindrical surface directed towards the rotor has two zones: one which performs the function of a labyrinth seal and one which performs a centring function. The leakage is thus used for centring the ring. There is therefore no hydrostatic bearing in the strict sense. In this proposal, there are major problems in the mounting of the ring on the casing, since, of course, a good moveability of the ring and a leak-tight connection must be achieved simultaneously. How this is to be solved satisfactorily is not stated. Other problems arise from the fact that, in the case of the low leakage to be sought after, centring can scarcely be achieved.